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Climatic and oceanological changes in the southwestern part of the Sea of Okhotsk during the last 94 kyr
Progress in Oceanography 179 (2019) 102215
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Climatic and oceanological changes in the southwestern part of the Sea of Okhotsk during the last 94 kyr
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Antonina V. Artemova , Yuriy P. Vasilenko, Sergey A. Gorbarenko, Aleksandr A. Bosin, Valentina V. Sattarova ⁎
V.I. Il’ichev Pacific Oceanological Institute FEB RAS, 43, Baltiyskaya Str., Vladivostok 690041, Russia
ARTICLE INFO
ABSTRACT
Keywords: Diatoms Ice-rafted Debris Late Pleistocene Holocene Ice conditions Environmental changes
This paper presents the results of a study on a sediment core obtained from the southwestern part of the Sea of Okhotsk. By analyzing the diatoms in the sediment, along with the productivity proxy and ice-rafted debris data, we reconstructed the regional environmental and climatic changes occurring over last 94 kyr (sea-ice expansion, periods of active ice melting, and variations in marine plankton productivity). Local warming and cooling events occurring in the Late Pleistocene and Holocene periods, as well as transgressive-regressive fluctuations in the sea level, were documented. The shift in the relatively warm interglacial conditions to severe glaciation over 34 kyr ago that occurred in the studied area of the Sea of Okhotsk was reported for the first time. The time at which the La Pérouse Strait opened, the Soya Current input began, and the establishment of modern hydrological conditions was approximately 5.6 kyr.
1. Introduction Owing to its high sensitivity to the global changes that occurred in the Late Pleistocene and Holocene periods, as well as its great influence on the oceanological conditions of the North Pacific, the Sea of Okhotsk is a key subject of paleoceanological research (Honjo, 1997). Deep-sea sediments provide reliable information on past climate changes as they contain the diatoms that have inhabited in the past and settled on the seabed. Diatoms are among the most informative paleontological groups, and serve as good markers of the paleo-conditions of surface water (salinity, temperature, ice conditions, currents, and the abundance of nutrients in the upper 50–100 m). Diatom analysis is a reliable approach to reconstructing the past climate. Previous studies on the diatomic flora of the Sea of Okhotsk focused on the species composition and seasonal development of phytoplankton (Jousé, 1962; Smirnova, 1959; Kiselev, 1947; Semina, 1974; Gorbatenko, 1997; Shuntov, 2001). The composition and features of the distribution of diatoms in the surface sediments of the Sea of Okhotsk have been systematically studied, which resulted in the development of the ecosystem approach to paleoecological studies (Jousé, 1962). In subsequent work, four complexes of diatoms in the surface sediments of the Sea of Okhotsk were identified by conducting cluster and factor analyses, which were related to the distribution of the main water masses (Sancetta, 1981, 1982). Tsoy et al. (2009) analyzed the species and
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Corresponding author. E-mail address: [email protected] (A.V. Artemova).
https://doi.org/10.1016/j.pocean.2019.102215
Available online 01 November 2019 0079-6611/ © 2019 Elsevier Ltd. All rights reserved.
ecological composition of diatoms, and their quantitative content, in the surface sediments of various underwater morphostructures. They separated the slope complexes of Sakhalin and Kamchatka, and highlighted the unique ice-neritic complex of the northern shelf of the Sea of Okhotsk. Shiga and Koizumi (2000) investigated the spatial distribution of the main types of diatoms in surface sediments, and linked the distribution of species to the duration of the ice season. They identified three ecological groups of diatoms: species-indicators of ice coverage, species-indicators of high productivity, and species-indicators of open ocean water. Kazarina and Yushina (1999) detected a relationship between the distribution of diatom species and oceanological parameters (temperature, salinity, currents, and productivity), and classified diatoms by temperature preference. Ren et al. (2014) summarized data collected from surface sediments and conducted statistical Q-mode factor analysis. They associated diatom assemblages with the Arctic, Subarctic, and Subtropical water masses, and observed a relationship between the diatom composition and sea surface temperature. The climate-stratigraphic method has been followed to separate Pleistocene sediments (Pushkar and Cherepanova, 2001, 2008; Polyakova, 1997), and identify complexes of diatoms corresponding to cooling and warming. The relationship between different groups of diatoms and the ice standing duration (Shiga and Koizumi, 2000) has been used to propose the distribution of and changes in ice cover in the Sea of Okhotsk over the past 21.5 kyr, and it has been found that the
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modern hydrological conditions of this region were established approximately 8 ka (Shimada et al., 2004, Okazaki et al., 2005). Recent studies on the paleoceanology and reconstruction of changes in the ice cover of the Sea of Okhotsk focused on changes in the paleoclimate, primary productivity, and the role of the ice cover in the development and variations of dominant microorganisms between glacial and interglacial periods (Okazaki et al., 2005; Sakamoto et al., 2006; Gorbarenko et al., 2010, 2012, 2014; Artemova et al., 2017, 2018; Sattarova and Artemova, 2015). However, the changes in diatom assemblages in response to changes in the southwestern Okhotsk Sea (SW OS) environment during the Late Pleistocene and Holocene periods have been poorly studied, despite the important role of diatoms as a sensitive proxy of variations of surface hydrology and sea ice coverage. Here, we report diatom assemblage data reflecting the S-W OS climate and environmental changes over the last 96 kyr, and correlate them with ice-rafted debris (IRD) data. The purpose of this work was to reconstruct the paleoceanographic conditions of the S-W OS during the Late Pleistocene and Holocene periods by analyzing the diatoms in a sediment core from the region. To achieve this goal, we studied the diatom assemblages that accumulated during the Late Pleistocene and Holocene periods (Marine Isotope Stages (MIS) 1-5). An age model for studied core was constructed, and the contents of CaCO3, total organic carbon (TOC), chlorin, and IRD were used to verify the diatom analysis results. Then the productivity and ice conditions of the S-W OS were reconstructed.
Standard 24 × 24-mm cover glasses and NORLAND synthetic gum with a refractive index of 1.56 were used to create permanent slides. The diatoms were identified and counted using a Micmed-6 microscope at a magnification of ×900. The number of diatom frustules was determined; and 300–350 frustules were identified for each slide to calculate the quantitative ratios of diatom taxa in the assemblages.
2. Oceanographic setting
CaCO3 = (TC-TOC) × 8.33
3.3. Chlorin content analysis The chlorin content of core samples was analyzed at the Pacific Oceanological Institute, Vladivostok, Russia (POI FEB RAS) using a Shimadzu UV-3600 spectrophotometer (each sample ca 1 g of dry sediment, every 1 cm, accuracy of up to 0.001 optical density units, wavelength of 665–666 nm) following the procedure proposed by Harris and Maxwell (1995). 3.4. Total organic carbon and CaCO3 analyses The total carbon (TC) and TOC contents of core samples collected every 2 cm were analyzed at the POI FEB RAS using an AN-752910N express analyzer. The CaCO3 and TC contents were determined following the acid and thermal decomposition (burning in a stream of oxygen) methods, respectively following the procedure reported by Gorbarenko et al. (1998). The CaCO3 content was calculated using a conversion factor for carbonate.
The Sea of Okhotsk is connected to the Pacific Ocean by numerous straits traversing the Kuril Island Ridge, the Sea of Japan by the La Pérouse Strait, and the Amursky Liman by the Nevelskoy and Tatar Straits. The Sea of Okhotsk is the coldest of the North West Pacific marginal seas. Overall, the current surface water circulation in the Sea of Okhotsk is cyclonic and, in general, consisted from the West Kamchatka Current, which carries warm Pacific water northward, and the East Sakhalin Current, which travels southward (Honjo, 1997; Fig. 1). In the Okhotsk Sea ice formation begins in November. The ice is thickest in March (0.8–1.0 m), and ice formation subsequently slows (Watanabe and Wakatsuchi, 1998; Martin et al., 1998). From April to June, the ice gradually melts and breaks apart. The hydrophysical and hydrochemical features of the surface water during spring create favorable conditions for phytoplankton blooms. The nutrient stock in surface layer forms in the Sea of Okhotsk during the autumn-winter convective mixing period, when deep water rises.
3.5. Oxygen and carbon isotopic ratios The isotopic compositions of oxygen and carbon (δ18O and δ13C) in the shells of Neogloboquadrina pachyderma sin., a species of planktonic foraminifera, were analyzed using a Finigan-MAT251 mass spectrometer without prior calcination following the standard technique in the laboratory of R. Tidemann (GEOMAR, Germany). 3.6. Magnetic susceptibility and moisture content The magnetic susceptibility and moisture content were measured every 2 cm along the length of the core during the cruise using an IMB-2 magnetic susceptibility kappameter with a special sensor for moisture (West Company Ltd., Kaliningrad). 3.7. Ice rafted debris
3. Materials and methods
IRD is the terrigenous component of sediment, i.e., it is the number of clastic grains in the 0.15–2.0 mm fraction per 1 g of dry sediment. Samples for determining the IRD content were collected every 3 cm. After wet sieving the sediment samples through a 0.15-mm sieve and the residue was dried and screened through a 2-mm sieve. The IRD content was measured in the remaining 0.15–2 mm sediment fraction. The IRD content analysis was described in detail by Vasilenko et al. (2019).
3.1. Materials Core GE 99-10-3 (48°18.7′N, 146°08′E) was obtained during the Russian-German cruise aboard the Pegasus Upland. The core was recovered from an area within the influence of the East Sakhalin Current at a depth of 1335 m, and had a length of 770 cm (Fig. 1). Lithological and micropaleontological analyses were conducted, and the magnetic susceptibility of the sediment, IRD, TOC, chlorin, and CaCO3, oxygen and carbon isotopic ratios, and the moisture content were determined following the methods described in this section.
3.8. Age model The age model of GE99-10-3 core was developed based on tephrochronology, the results of the δ18O analysis of foraminifera shells, the variations in magnetic susceptibility and moisture content, and the variations in the paleoproductivity proxies (chlorin, TOC, and CaCO3 contents; Fig. 2). Furthermore, three volcanic ash layers (K3, T, and Aso4;Derkachev et al., 2004, 2016; Derkachev and Portnyagin, 2013) were identified in the core, which were used as key age points (Figs. 2 & 3, Table 1).
3.2. Diatom analysis The diatoms in the sediments were insulated following a method developed at the Shirshov Institute of Oceanology RAS, which involves the use of a heavy potassium-cadmium liquid containing cadmium iodide (CdI2) and potassium iodide (KI) with the following ratio: H2O:CdI2:KI = 1:2.5:2.25 (Jousé, 1962; Diatoms of the USSR, 1974). 2
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Fig. 1. The location of core GE99-10-3. The blue arrows represent the surface currents, based on Chernyavskiy (1981). Numbers are standing for the follow straits: 1 Nevelskoy Strait, 2 - Tatar Strait, and 3 - La Pérouse Strait. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3.8.1. Position of boundaries between different marine isotopic stages The onset of MIS 1 (11.7 ka) was established according to the extreme lightening of the δ18O composition of planktonic foraminifera shells, the increasing contents of chlorin and TOC, and changes in the proxy parameters, which are characteristic for the onset of Holocene (Gorbarenko, 1996; Gorbarenko et al., 2002a,b, 2004; Figs. 2 & 3, Table 1). The onset of MIS 2 was stratigraphically located above the volcanic ash layer K3 and established following changes in the δ18O composition of planktonic foraminifera shells and the productivity proxies values (Fig. 2). The onset of MIS 3 corresponds to the transition from relatively low paleoproductivity proxies values during MIS 4 to relatively high values during MIS 3 (Fig. 2). The sharp peak in the chlorin content, which is the result of the increasing productivity during Dansgaard–Oeschger interstadial (DOI) 17, allows the position of this boundary to be elucidated (Fig. 2). The onset of MIS 4 was also determined by the general pattern of paleoproductivity proxies, particularly the transition from relatively high values during MIS 5 to relatively low values during MIS 4 (Fig. 2).
The division of MIS 1 into the period of early Holocene (11,7–6 ka) with dramatic changes in environmental conditions and sedimentation of the Sea of Okhotsk and the period of the late Holocene (6–0 ka) with relatively stable conditions (Gorbarenko, 1996; Gorbarenko et al., 2002a, 2002b; Barash et al., 2001). The boundary between these two time intervals is the sharp increase in the content of diatoms in the sediments of the Sea of Okhotsk with the formation of diatom ooze. The age of this boundary in the Sea of Okhotsk is defined as 6 ka and welldated (Gorbarenko, 1996; Gorbarenko et al., 2002ab, 2004, 2010). 3.8.2. Identification of the Dansgaard–Oeschger interstadials Gorbarenko et al. (2007, 2010, 2012) have demonstrated that the productivity of the Sea of Okhotsk increased simultaneously with the DOI and decreased during Dansgaard–Oeschger stadial (DOS). Thus, the productivity of the Sea of Okhotsk clearly responds to changes in the global climate on the millennial time scale, which was also reported in a study on ice cores collected in Greenland (Dansgaard et al., 1993). This allowed us to graphically correlate the productivity proxy records 3
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Fig. 2. Correlation of the changes in the productivity proxies (CaCO3, TOC, and chlorin contents), planktonic foraminifera δ18O record, and the moisture content and magnetic susceptibility of core GE99-10-3 with depth, along with the NGRIP δ18O record on the GICC05 age scale in the upper graph (North Greenland Ice Core Project Members, 2004; Wolff et al., 2010). The gray band denotes the position of the volcanic ash layers (K3, T, and Aso4). The ‘I’ terms with numbers in the upper section indicate the positions of the Dansgaard–Oeschger interstadials.
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Fig. 3. Age-Depth diagram for core GE 99-10-3. The ‘DOI’ terms with numbers in the upper section indicate the positions of the Dansgaard–Oeschger interstadials.
Table 1 Control points for the age model of core GE99-10-3. Control point
Dating method
Depth (cm)
Age (ka)
6 ka Onset of Onset of Onset of DOI 2 Onset of DOI 5 K3 DOI 6 DOI 7 T DOI 8 DOI 11 DOI 12 DOI 13 DOI 14 DOI 17 Onset of DOI 18 DOI 19́
Multiproxy analysis Multiproxy analysis Multiproxy analysis Multiproxy analysis Multiproxy analysis Multiproxy analysis Multiproxy analysis Volcanic ash layer Multiproxy analysis Multiproxy analysis Volcanic ash layer Multiproxy analysis Multiproxy analysis Multiproxy analysis Multiproxy analysis Multiproxy analysis Multiproxy analysis Multiproxy analysis Multiproxy analysis Multiproxy analysis Multiproxy analysis Multiproxy analysis Multiproxy analysis Multiproxy analysis Volcanic ash layer Multiproxy analysis
60 117 127 137 196 218 241 246–250 256 268 274–277 293 322 349 361 382 423.5 424 463 514 540 555 581 667 697–700 726
6 11.7 12.7 14.7 23.34 28 32.5 33 33.74 35.48 36.5 38.22 43.34 46.86 49.28 54.22 59.44 59.5 64.095 69.55 72.33 73.9 76.45 84.65 88 89.95
MIS 1 Younger Dryas (YD) Bølling-Allerød warming (B/A) MIS 2
MIS 3
DOI 19 Onset of MIS 4 DOI 20 DOI 21 Aso4 DOI 22
Note. DOI – Dansgaard–Oeschger interstadial.
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with that of the NGRIP δ18O content (North Greenland Ice Core, 2004), which provides a reliable tool for building high-resolution chronostratigraphy. The interstadials of long-lasting DOI cycles, which are before cycles 1, 8, 12, 14, 19, 20, 21, and 22 (Solé et al., 2007), are clearly established within the corresponding MIS and indicated by the peak values of the productivity proxies (chlorin, TOC) and moisture content, and the decrease in the magnetic susceptibility (Fig. 2). In addition to the above parameters, DOI 1 (onset of Bølling-Allerød (B/A) warming) was identified by the increase in the CaCO3 content (Fig. 2). The peak in the CaCO3 content is typical for Okhotsk Sea sediments that accumulated during B/A warming (Gorbarenko et al., 2002a, b). Additional age markers between the control points were obtained based on the graphical correlation of the chlorin content and magnetic susceptibility of the DO cycles in the δ18O NGRIP record (North Greenland Ice Core, 2004). Furthermore, increases in the TOC, CaCO3, and chlorin content and the minimum magnetic susceptibility values correlated with the peaks in the δ18O content corresponding to certain DOI (2, 5, 6, 7, 11, 13, 17, 18) (Fig. 2).
predominant: A. curvatulus (2.35–9%), R. hebetata f. hiemalis (0.9–15.9%), S. latimarginatus (17.3–2.3%), T. longissima (2.35–10%). The content of the tyhopelagic species Paralia sulcata also increased (3.8–28.9%). T. gravida (2.4–25%) was predominant in the near-ice group, and its proportion in this assemblage was the highest downcore (9–32%), followed by Bacterosira bathyomphala (3–11%). The proportion of sea-ice species (up to 10%), including Thalassiosira kryophila, Thalassiosira hyaline, Thalassiosira nordenskioeldii, Fragilariopsis cylindrus, and Fragilariopsis oceanica, also increased. Extinct redeposited diatoms were also founded in this assemblage. In the range of 24–18 kyr, the number of diatom frustules and proportion of the oceanic group in the sediment increased. During 18–12 kyr, the number of diatom frustules decreased. The content of S. latimarginatus increased, reaching 12.7–17.3% (Fig. 4). Assemblage 1 (since 11.7 ka) exhibited the highest abundance of diatom frustules (2.4–20.5 × 106frustules/1 g) with high species diversity. The oceanic species S. latimarginatus (5–18%), C. marginatus (13–35%), Neodenticula seminae (3.1–24.5%), and A. curvatulus (1–13%) were prevalent. The warm-water species Shionodiscus oestrupii (0.7–3.8%) and Thalassiosira pacifica (0.5–1.6%) were also observed. The share of oceanic species increased in accordance with the increasing diatom frustule content. The warm-water Japan Sea endemic species T. nitzhioides appeared since 5.6 ka. The share of sea-ice and near-ice diatoms decreased, and the content of P. sulcata was very small.
4. Results The following oceanic species were predominant in assemblage 5 (94–73.9 kyr): Shionodiscus latimarginatus (12–35%), Thalassiothrix longissima (10–18.5%), Coscinodiscus marginatus (12.5–33%), Rhizosolenia hebetata f. hiemalis (5.7–19.6%), and Actinocyclus curvatulus (3.4–20.5%). The diatom frustules content was 0.56–7.0 × 106/1 g (Figs. 4, 5). The content of near-ice species reached 7%, which was mainly due to the proportion of Thalassiosira gravida. Sea-ice flora are absent from this assemblage. The taxonomic composition of diatoms in assemblage 5 did not change significantly. However, the diatom frustule content fluctuated significantly. Three subassemblages can be identified based on changes in the concentrations of diatoms. In subassemblage 5c (94–93 kyr), the diatom content reached 2.5 × 106 frustules/1 g, and the proportion of oceanic species is small due to the high content of T. gravida (Fig. 4). In subassemblage 5b (93–85 kyr), the diatom content was lower than that of the previous assemblage (0.56–2.5 × 106 frustules/1 g). The proportion of oceanic species (35–65%) is lower than that of subassemblage 5c due to the presence of T. gravida (up to 7%) (Fig. 4, Plate 1). In subassemblage 5a (85–73.9 kyr), the increased diatom content of the sediment (2.8–7 x106 frustules/1 g) indicated that the conditions were more favorable for diatoms, and the proportion of oceanic species reached 92% (Fig. 4). The number of diatoms in assemblage 4 (73.9–59.5 kyr) was lower (0.19–1.12 x106 frustules/1 g). The diatom assemblages mainly contain cold-water oceanic species, such as R. hebetata (25–27%), T. longissima (6.6–25.6%), and S. latimarginatus (up to 21%). The percentage of nearice and sea-ice was 6% around 73.9–71.5 kyr, which continuously increased and reaching a maximum value of 21.8% at the 59.5 ka. However, its percentage sharply decreased about 69 ka. T. gravida is the predominant near-ice species. However, the content of cryophilic flora in assemblage 4 is insignificant (Fig. 4). In assemblage 3 (59.5–28 kyr), the number of diatoms increases (1.06–3.6 x106 frustules/1 g), along with the concentration of oceanic species increased. The number of near-ice diatoms decreased, constituting 2.2–10.4% of the total diatoms in the assemblage. The dominant species of this assemblage were A. curvatulus (16.4–3.4%), R. hebetata f. hiemalis (6.2–20.9%), C. marginatus (13–37%), S. latimarginatus (2.3–25.7%), and T. longissima (6.8–25.9%) (Fig. 4). Assemblage 2 (28–11.7 kyr,) is characterized by a sharp decrease in the number of diatom frustules per gram (up to 0.35 x106 frustules/1 g). The proportion of oceanic species is 35.7–70%, which is dominated by C. marginatus (13–37%). The following oceanic species were also
5. Discussion 5.1. End of MIS 5c-MIS 5b The low abundance and poor species composition of diatoms in the sediments during this period, in comparison with those of modern sediments, indicate that the climatic conditions were unfavorable at this time. The presence of arctic-boreal (including near-ice) species indicates a longer period of standing sea ice. The sea ice melted during summer, as indicated by the presence of P. sulcata in the assemblages (Figs. 4, 5). Predominance of typical Okhotsk cold-favoring species and high proportion of the near-ice group indicate that cold water masses prevailed in the S-W SO. Additionally, the climate was also cold. Based on the δ13C data, we can conclude that the terrigenous component was abundant in the supply of organic matter. The low contents of TOC and CaCO3 in the sediment also suggest low marine bio-productivity. However, according to the comparison of the IRD data collected for this period and the Holocene, the ice conditions were moderate and the winters were relatively mild. Within this period, the climate was cold, the sea regressed, and heavy terrigenous material was supplied to the sediments (Fig. 5). 5.2. MIS 5a During this period, the number of diatoms frustules, species diversity, and number of oceanic species increased. Moderately warm species were mainly present. The decrease in the percentage of sea-ice species indicates a reduction in the length of the winter seasons. The δ13C (Fig. 5) data suggest that marine productivity was high. During 84.5–81.5 kyr, the increasing proportion of R. hebetata f. hiemalis and TOC content indicated favorable climatic conditions with increased salinity and productivity in the surface waters of this area (Fig. 5). 5.3. MIS 4 The sediments of this period were characterized by low diatom abundance. The production of diatoms decreased due to the longer icecoverage season. The onset of glaciation conditions was gradual, which affected the ecological composition of diatoms. In the beginning of this 6
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Fig. 4. Distribution of the dominant species of diatoms, and the diatom content in core GE 99-10-3. The pink and dark blue bands denote periods with warm and cold environmental conditions, respectively. The light blue band denotes the period with slightly warmer environmental conditions during MIS 2. The red line marks the time at which the modern hydrological conditions began. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 5. Changes in the paleoproductivity proxies and environmental conditions of the southwestern region of the Sea of Okhotsk over time based on changes in the diatoms content in the sediment, and ratio of ecological groups, and the IRD, CaCO3, TOC, δ13C, and chlorin contents. The pink, dark blue, and light blue bands, and red line indicate the same periods as those in Fig. 4. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
period, oceanic flora was predominant. From 72 kyr, the diatom assemblages were dominated by sea-ice and near-ice species, particularly cryophilic species, which proliferated both in the ice and near the ice edge. This suggests the occurrence of relatively severe ice conditions with longer sea-ice standing seasons. This assumption is confirmed by the increase in the content of , decrease in the supply of marine
organics, and increase in the proportion of allochthonous bioorganic inputs, as indicated by the δ13C value. The presence of the brackish-water species P. sulcata, which reached a subdominant position in the diatom assemblages, suggests the occurrence of active warming and ice-melting periods. Generally, the data indicated a cooling climate. The peak content of extinct species 8
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Plate 1. Diatom assemblages in the sediments from the southwestern part of the Sea of Okhotsk. 1–Actinocyclus ochotensis Jousé. 2–Actinocyclus curvatulus Janisch. 3– Thalassiosira gravida Cleve, Shionodiscus latimarginatus (Makarova) Alverson. 4–Rhizosolenia hebetata Bailey. 5–Bacterosira bathyomphala (Cleve) Syvertsen & Hasle. 6– Coscinodiscus oculus-iridis (Ehrenberg) Ehrenberg. 7–Shionodiscus latimarginatus (Makarova) Alverson. 8–Thalassiosira gravida Cleve. 9– Coscinodiscus marginatus Ehrenberg. 1011– Neodenticula seminae (Simonsen et Kanaya) Akiba et Yanagisawa. 12– Thalassiothrix longissima Cleve et Grunow. 13–Thalassionema nitzschioides (Grunow) Mereschkowskii.
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indicated that the sea level regressed. The absence of Japanese Sea species point to that there was no interaction between the Seas of Okhotsk and Japan.
environment and the enhancement of primary bioproductivity. However, as these conditions are unfavorable for the diatoms, there were few diatom products. The high bioproductivity was attributed to the development of carbonate microfossils.
5.4. MIS 3
5.7. Younger Dryas
The increase in the number of diatom frustules and content of oceanic species in the sediments indicates that, in this period, the Sea of Okhotsk experienced different climatic conditions. The ecological composition of diatoms at this time reflects a relatively warm climate, however, interglacial environmental conditions were not reached. The TOC content indicates that the supply of organic material to the bottom was slightly higher than that in the previous cold period. The lightening δ13C isotopes also indicate an increased supply of autochtonous organics. The number of near-ice and sea-ice diatoms decreased, reflecting a decrease in the ice coverage of the S-W OS. The salinity of the surface layer may have been higher than it is today. During this period, two regional warming events occurred at 58–49 and 45–36 kyr. The warming, accompanied by active ice melting, increased the diatom vegetation period. The lightening of the δ13C isotope composition indicates an increase in the supply of marine organics. Furthermore, the absence of CaCO3 confirms the predominance of diatoms in the phytoplankton (Fig. 5). From 36 to 28 ka, the number of diatoms decreased, the proportion of oceanic species sharply reduced, and the proportion of near-ice flora increased. Severe climatic conditions occurred, as indicated by the decrease in all productivity proxies, as well as the significant increase in the IRD content during this period (Fig. 5). Our data suggest that the severe climatic conditions, which are usually noted as characteristic of MIS 2, began earlier in this region, commencing 34 ka.
During this period, the cooling event occurred. This was reflected in the increase in the contents of ice and glacial species in the diatom complex. The number of diatoms was low, which was because the previous deglacial warming was interrupted by strong, short-term cooling, and an increase in the duration of the winter season. The TOC content decreased alongside the decrease in the concentrations of chlorin and CaCO3, confirming the return to glacial and severe environmental conditions, and a decrease in primary productivity (Fig. 5). 5.8. Early Holocene (11.7–8.3 kyr) This period marked the beginning of Holocene warming (Termination 1B). The concentration of near-ice species in the glacial assemblages decreased, and the proportion of sea-ice diatoms was lowest. This was due to the significant warming and decrease in the extent of sea ice. The low abundance of diatoms was likely due to the low input of nutrients, and their growth was limited by the low amount of silicon. An interesting local event occurred at 11.7–11 kyr, which was associated with the strong desalination of the sea surface layer due to the increased ice melting. This was marked by the synchronous peaks in the growth of P. sulcata and the IRD content. The sharp increase in the proportion of R. hebetata following this event indicated an increase in salinity in the study area until 6 ka.
5.5. MIS 2
5.9. The last 8.3 kyr
The severely cold climate conditions and low bio-productivity continued until 24.5 ka. From 24.5 to 18 ka, the composition of diatoms reflects some environmental warming in the S-W OS. The number of diatom frustules and the proportion of oceanic species both increased. According to our diatom data, from 18 to 15 ka, the hydrological conditions were significantly different to those observed today. The diatom assemblages were unique and have not been found in modern deep-water sediments from the Sea of Okhotsk. Diatom assemblages with glacial species, including B. bathyomphala, Thalassiosira antarctica, and T. gravida, and cryophilic species are typical for sediments in the Arctic seas (Polyakova, 1996; Tsoy et al., 2009). The diatom complex indicates prolonged sea ice standing, a reduction in the length of summer seasons, and temperature conditions close to those currently observed in the Arctic regions. The long sea ice-standing seasons, decrease in the input of the both Pacific waters and Amur River inflow, which provides a large amount of necessary for the development of diatoms nutrients to the Sea of Okhotsk (Seki et al., 2004a, 2004b), reduced the productivity of diatoms. The sharp decline in CaCO3 and TOC confirms the occurrence of severe glaciation conditions, known as the cooling event Heinrich 1 (Gorbarenko et al., 2014). The increase in the volumes of sea ice was confirmed by the spike in the IRD record. The decrease in the sea level was reflected in the appearance of redeposited diatom species from the eroded, older Cenozoic sediments.
This period is characterized by the rapid development of diatom flora and significant increase in their abundance. The increased diatom production, flow of organic matter, and nutrient inputs from land were confirmed by the chlorin and δ13C data. During this period, the hydrological conditions of the Sea of Okhotsk were turned to those experienced today. The resumption of the interactions between the Seas of Japan and Okhotsk, and the influx of warm and saline waters from the Soya Current were indicated by the presence of Thalassionema nitzschioides in the sediment, which migrated from the Sea of Japan approximately 5.6 ka, later then mentioned in Shimada et al. (2004) for off Hokkaido Island. The appearance of N. seminae over the last 6 kyr indicates the enhanced input of Pacific surface water with the onset of modern Sea of Okhotsk hydrology. 6. Conclusions The diatom species composition and content of diatom frustules in the sediments of the S-W OS, during the Holocene and Late Pleistocene periods reflect the global and regional climatic and environmental changes over the last 94 kyr. During studied period prolonged sea icestanding season and extending sea-ice cover led to lowered diatoms productivity and vice versa. Sea level regression led to the absence of interactions between the Seas of Okhotsk and Japan about 75 ka. During 58–49 and 45–36 kyr, regional active warming events occurred with higher productivity and an increase in the supply of marine organics. During 36–28 kyr, severe climatic conditions were experienced, and the productivity of marine phytoplankton and influx of organic matter from the shore were low. Local cooling began in 34 kyr. From 18 to 15 kyr, a very long season with an ice-covered sea occurred, and the environmental conditions of this region were similar to those currently observed in the Arctic. The sea level was also significantly lower, which is likely due to climate aridization and terrestrial glaciation. During B/A, a period of warming and active ice melting with a
5.6. Bølling-Allerød During this period, the warming, the longer summer seasons, and the melting of great volumes of sea ice were reflected in the growth of oceanic species and the increased share of the brackish-water, lowsalinity-tolerant species P. sulcata (reaching 28.9%, the highest in studied sediments). During this period, there were sharp and significant peaks in the contents of productivity indicators (TOC, chlorin, and CaCO3). This confirms the favorable climatic conditions of the 10
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great flood and desalination of seawater was experienced alongside an increase in productivity. During YD, glacial conditions returned, while significant warming occurred from 11.7 to 8.3 kyr. During 11.7–11 kyr, large volumes of floating ice began to drift into the study area and the seawater was strongly desalinated, which prevents the development of diatom flora. In the last 8.3 kyr, the productivity, organic matter content and nutrient inputs increased. Modern hydrological conditions were established 5.6 kyr, water exchange between the neighboring basins of the Seas of Japan and Okhotsk commenced, and the influx of Pacific water also increased.
Gorbarenko, S.A., Khusid, T.A., Basov, I.A., Oba, T., Southon, J.R., Koizumi, I., 2002a. GlacialHolocene environment of the southeast Okhotsk Sea: evidence fromgeochemical and paleontological data. Palaeogeogr. Palaeoclimatol. Palaeoecol. 177 (3–4), 237–263. Gorbarenko, S.A., Nürnberg, D., Derkachev, A.N., Astakhov, A.S., Southon, J.R., Kaiser, A., 2002b. Magnetostratigraphy and tephrochronology of the upper Quaternary sediments in the Okhotsk Sea: implication of terrigenous, volcanogenic and biogenic matter supply. Mar. Geol. 183, 107–129. Gorbarenko, S.A., Psheneva, O.Y., Artemova, A.V., Matul', A.G., Tiedemann, R., Nurnberg, D., 2010. Paleoenvironment changes in the NW Okhotsk Sea for the last 18kyr determined with micropaleontological, geochemical, and lithological data. Deep-Sea Res. Part I: Oceanogr. Res. Papers 57 (6), 797–811. Gorbatenko, K.M., 1997. Composition, structure and dynamic of plankton from Sea of Okhotsk. Tesis of Ph.D., IBS FEB RAS, Vladivostok, 24 (In Russian). Gorbarenko, S.A., Southon, J.R., Keigwin, L.D., Cherepanova, M.V., Gvozdeva, I.G., 2004. Late Pleistocene–Holocene oceanographic variability in the Okhotsk Sea: geochemical, lithological and paleontological evidence. Palaeogeogr. Palaeoclimatol. Palaeoecol. 209 (1–4), 281–301. Harris, P.G., Maxwell, J.R., 1995. A novel method for the rapid determination of chlorin concentrations at high stratigraphic resolution in marine sediments. Org. Geochem. 23, 853–856. Honjo, S., 1997. The Northwestern Pacific Ocean, a crucial ocean region to understand global change: rational for new international collaborative investigations. In: Tsunogai, S. (Ed.), Biogeochemical Processes in the North Pacific. Japan Marine Science Foundation, Tokyo, pp. 233–248. Jousé, A.P., 1962. Stratigraphic and paleogeographic studies in the Northwestern Pacific. Nauka, Moscow In Russian. Kazarina, G.Kh., Yushina, I.G., 1999. Diatom in Recent and fossil sediments of the Okhotsk Sea. Berichte zur Polarforschung 134–148. Kiselev, I.A., 1947. Phytoplankton of the Far Eastern seas as an indicator of some features of their hydrological regime. Trudy GOIN 1 (13), 189–212 In Russian. Martin, S., Drucker, R., Yamashita, K., 1998. The production of ice and dense shelf water in the Okhotsk Sea polynyas. J. Geophys. Res. Oceans 103 (12). North Greenland Ice Core Project Members, 2004. High-resolution record of Northern Hemisphere climate extending into the last interglacial period. Nature 431, 147–151. Okazaki, Y., Takahashi, K., Katsuki, K., et al., 2005. Late Quaternary paleoceanographic changes in the southwestern part of the Okhotsk Sea: based on analyses of geochemical, radiolarian, and diatom records. Deep Sea Res. II 52 (16–18), 2332–2350. Polyakova, Y., 1996. Diatoms of the Eurasian Arctic Seas and their distribution in surface sediments. Rep. Polar Res. 212, 315–325. Polyakova, I. Ye, 1997. The Eurasian Arctic Seas during the Late Cenozoic. Scientific World, Moscow. Pushkar, V.S., Cherepanova, M.V., 2001. Diatoms of Pliocene and Antropogene of the North Pacific (stratigraphy and Paleoecology). Dalnauka, Vladivostok. Pushkar, V.S., Cherepanova, M.V., 2008. Diatom assemblages and Quaternary deposits correlation of the North Western Pacific. Dalnauka, Vladivostok. Ren, J., Gersonde, R., Esper, O., Sancetta, C., 2014. Diatom distributions in northern North Pacific surface sediments and their relationship to modern environmental variables. Palaeogeogr. Palaeoclimatol. Palaeoecol. 402, 81–103. Sakamoto, T., Ikehara, M., Kanamatsu, T., et al., 2006. Millennial-scale variations of sea-ice expansion and their relation to Okhotsk Intermediate Water formation in the southwestern part of the Okhotsk Sea during the past 120 kyr: Results from IMAGES core MD01-2412 analyses. Global Planet. Change 53, 58–77. Sancetta, C., 1981. Oceanographic and ecological significance of diatoms in surface sediments of the Bering and Okhotsk seas. Deep-Sea Res. 28A, 789–817. Sancetta, C., 1982. Distribution of diatom species in surface sediments of the Bering and Okhotsk seas. Micropaleontology 28, 221–257. Sattarova, V.V., Artemova, A.V., 2015. Geochemical and micropaleontological character of deep-sea sediments from the Northwestern Pacific near the Kuril-Kamchatka Trench. Deep Sea Res. Part II: Top. Stud. Oceanogr., pp. 111–118. Seki, O., Ikehara, M., Kawamura, K., 2004a. Reconstruction of plaeoproductivity in the Sea of Okhotsk over the last 30 kyrs. Paleoceanography 19. https://doi.org/10.1029/ 2002PA000808. Seki, O., Kawamura, K., Ikehara, M., 2004b. Variation of alkenone sea surface temperature in the Sea of Okhotsk over the last 85 kyrs. Org. Geochem. 35, 347–354. Semina, G.I., 1974. Phytoplankton of the Pacific Ocean. Science, Moskow in Russian. Shiga, K., Koizumi, I., 2000. Latest Quaternary oceanographic changes in the Okhotsk Sea based on diatom records. Mar. Micropaleontol. 38, 91–117. Shimada, C., Ikehara, K., Tanimura, Y., Hasegawa, S., 2004. Millennial-scale variability of Holocene hydrography in the southwestern Okhotsk Sea: diatom evidence. The Holocene 14 (5), 641–650. Shuntov, V.P., 2001. Biology of the Far Eastern seas. TINRO-Center, Vladivostok in Russian. Smirnova, L.I., 1959. Phytoplankton of the Sea of Okhotsk and Prikurilsky District. Acad. Sci. USSR Moscow. T in Russian. Solé, J., Turiel, A., Llebot, J.E., 2007. Classification of Dansgaard-Oeschger climatic cycles by the application of similitude signal processing. Phys. Lett. A 366, 184–189. Tsoy, I.B., Obrezkova, M.S., Artemova, A.V., 2009. Diatoms in surface sediments of the Sea of Okhotsk and the Northwest Pacific Ocean. Oceanology 49, 130–139. Vasilenko, Y.P., Gorbarenko, S.A., Bosin, A.A., Artemova, A.V., Yanchenko, E.A., Shi, X.-F., Zou, J.-J., Liu, Y.-G., Toropova, S.I., 2019. Orbital-scale changes of sea ice conditions of Sea of Okhotsk during the last glaciation and the Holocene (MIS 4–MIS 1). Palaeogeogr. Palaeoclimatol. Palaeoecol. 533, 109284. https://doi.org/10.1016/j.palaeo.2019.109284. Watanabe, T., Wakatsuchi, M., 1998. Formation of 26.8-26.9 Ca water in the Kuril Basin of the Sea of Okhotsk as a possible origin of North Pacific Intermediate Water. J. Geophys. Res. 103, 2849–2865. Wolff, E.W., Chappellaz, J., Blunier, T., Rasmussen, S.O., Svensson, A., 2010. Millennial-scale variability during the last glacial: the ice core record. Quat. Sci. Rev. 29, 2828–2838.
Declaration of Competing Interest We declare that we have no conflict of interest. Acknowledgements The authors are extremely grateful to Dr. M. Malyutina and Prof. Dr. A. Brandt for the invitation to join the expedition KuramBio II supported by BMBF Grant no. 03G0250A to Angelika Brandt. This work was supported by the Russian Foundation for Basic Research (grant no. 19-05-00663 A), the project of the Ministry of Science and Education of Russia (grant no. АААА- А17-117030110033-0). This is KuramBio publication # 59. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.pocean.2019.102215. References Artemova, A.A., Gorbarenko, S.A., Vasilenko, Yu.P., Shi, X., Liu, Y., Chen, M.-T., 2017. Paleoceonography changes in the Okhotsk Sea during Late Pleistocene and Holocene according to diatoms. Quat. Int. 459, 175–186. Artemova, A.V., Sattarova, V.V., Vasilenko, Yu.P., 2018. Diatom distribution and geochemical evidence in the Holocene Kurile Basin sediments (Sea of Okhotsk). Deep Sea Research Part II: Topical Studies in Oceanography. https://doi.org/10.1016/j.dsr2.2017.12.019. Barash, M.S., Bubenshchikova, N.V., Kazarina, G.Kh., Khusid, T.A., 2001. Paleoceanograpfic studies in the central part of the Okhotsk Sea during the last 200 kyr (on the basis of micropaleontological data). Okeanologiya 41 (5), 755–767 in Russian. Chernyavskiy, V.I., 1981. Circulation systems of the Okhotsk sea. Izvestiya TINRO 105, 13–19 in Russian. Dansgaard, W., Johnsen, S.J., Clausen, H.B., Dahl-Jensen, D., Gundestrup, N.S., Hammer, C.U., Hvidberg, C.S., Steffensen, J.P., Sveinbjörnsdottir, A.E., Jouzel, J., Bond, G., 1993. Evidence of general instability of past climate from a 250-kyr ice-core record. Nature 364, 218–220. Derkachev, A.N., Nikolaeva, N.A., Gorbarenko, S.A., Portnyagin, M.V., Ponomareva, V.V., Nürnberg, D., Sakamoto, T., Iijima, K., Liu, Y., Shi, X., Lv, H., Wang, K., 2016. Tephra layers of in the quaternary deposits of the Sea of Okhotsk: Distribution, composition, age and volcanic sources. Quatern. Int. 425 (15), 248–272. Derkachev, A.N., Portnyagin, M.V., 2013. Marker tephra layers in the late quaternary deposits of the Sea of Okhotsk as evidence of catastrophic eruptions in the Nemo caldera complex (Onekotan Island, Kuril Islands). Stratigr. Geol. Correl. 21 (5), 553–571. Derkachev, A.N., Nikolaeva, N.A., Gorbarenko, S.A., 2004. The peculiarities of supply and distribution of clastogenic material in the Sea of Okhotsk during Late Quaternary. Russ. J. Pacif. Geol. 23 (1), 37–52. Diatoms of the USSR (fossil and modern), Nauka, Leningrad, 1974 (In Russian). Gorbarenko, S.A., 1996. Stable isotope and lithological evidence of late-glacial and Holocene oceanography of the Northwestern Pacific and its marginal seas. Quat. Res. 46, 230–250. Gorbarenko, S.A., Artemova, A.V., Goldberg, E.L., Vasilenko, Y.P., 2014. The response of the Okhotsk Sea environment to the orbital-millennium global climate changes during the Last Glacial Maximum, deglaciation and Holocene. Global Planet. Change 116, 76–90. Gorbarenko, S.A., Chekhovskaya, M.P., Souton, J.R., 1998. Detailed environmental changes of the Okhotsk Sea central part during Last Glaciation Holocene. Okeanologiya 38 (2), 305–308 In Russian. Gorbarenko, S.A., Goldberg, E.L., Kashgarian, M., Velivetskaya, T.A., Zakharkov, S.P., Pechnikov, V.S., Bosin, A.A., Psheneva, O.Yu., Ivanova, E.D., 2007. Millennium scale environment changes of the Okhotsk Sea during last 80 kyr and their phase relationship with global climate changes. J. Oceanogr. 63 (4), 609–623. Gorbarenko, S.A., Harada, N., Malakhov, M.I., Velivetskaya, T.A., Vasilenko, Y.P., Bosin, A.A., Derkachev, A.N., Goldberg, E.L., Ignatiev, A.V., 2012. Responses of the Okhotsk Sea environment and sedimentology to global climate changes at the orbital and millennial scale during the last 350kyr. Deep-Sea Res. II 61–64, 73–84.
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Climatic and oceanological changes in the southwestern part of the Sea of Okhotsk during the last 94 kyr
T
Antonina V. Artemova , Yuriy P. Vasilenko, Sergey A. Gorbarenko, Aleksandr A. Bosin, Valentina V. Sattarova ⁎
V.I. Il’ichev Pacific Oceanological Institute FEB RAS, 43, Baltiyskaya Str., Vladivostok 690041, Russia
ARTICLE INFO
ABSTRACT
Keywords: Diatoms Ice-rafted Debris Late Pleistocene Holocene Ice conditions Environmental changes
This paper presents the results of a study on a sediment core obtained from the southwestern part of the Sea of Okhotsk. By analyzing the diatoms in the sediment, along with the productivity proxy and ice-rafted debris data, we reconstructed the regional environmental and climatic changes occurring over last 94 kyr (sea-ice expansion, periods of active ice melting, and variations in marine plankton productivity). Local warming and cooling events occurring in the Late Pleistocene and Holocene periods, as well as transgressive-regressive fluctuations in the sea level, were documented. The shift in the relatively warm interglacial conditions to severe glaciation over 34 kyr ago that occurred in the studied area of the Sea of Okhotsk was reported for the first time. The time at which the La Pérouse Strait opened, the Soya Current input began, and the establishment of modern hydrological conditions was approximately 5.6 kyr.
1. Introduction Owing to its high sensitivity to the global changes that occurred in the Late Pleistocene and Holocene periods, as well as its great influence on the oceanological conditions of the North Pacific, the Sea of Okhotsk is a key subject of paleoceanological research (Honjo, 1997). Deep-sea sediments provide reliable information on past climate changes as they contain the diatoms that have inhabited in the past and settled on the seabed. Diatoms are among the most informative paleontological groups, and serve as good markers of the paleo-conditions of surface water (salinity, temperature, ice conditions, currents, and the abundance of nutrients in the upper 50–100 m). Diatom analysis is a reliable approach to reconstructing the past climate. Previous studies on the diatomic flora of the Sea of Okhotsk focused on the species composition and seasonal development of phytoplankton (Jousé, 1962; Smirnova, 1959; Kiselev, 1947; Semina, 1974; Gorbatenko, 1997; Shuntov, 2001). The composition and features of the distribution of diatoms in the surface sediments of the Sea of Okhotsk have been systematically studied, which resulted in the development of the ecosystem approach to paleoecological studies (Jousé, 1962). In subsequent work, four complexes of diatoms in the surface sediments of the Sea of Okhotsk were identified by conducting cluster and factor analyses, which were related to the distribution of the main water masses (Sancetta, 1981, 1982). Tsoy et al. (2009) analyzed the species and
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Corresponding author. E-mail address: [email protected] (A.V. Artemova).
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ecological composition of diatoms, and their quantitative content, in the surface sediments of various underwater morphostructures. They separated the slope complexes of Sakhalin and Kamchatka, and highlighted the unique ice-neritic complex of the northern shelf of the Sea of Okhotsk. Shiga and Koizumi (2000) investigated the spatial distribution of the main types of diatoms in surface sediments, and linked the distribution of species to the duration of the ice season. They identified three ecological groups of diatoms: species-indicators of ice coverage, species-indicators of high productivity, and species-indicators of open ocean water. Kazarina and Yushina (1999) detected a relationship between the distribution of diatom species and oceanological parameters (temperature, salinity, currents, and productivity), and classified diatoms by temperature preference. Ren et al. (2014) summarized data collected from surface sediments and conducted statistical Q-mode factor analysis. They associated diatom assemblages with the Arctic, Subarctic, and Subtropical water masses, and observed a relationship between the diatom composition and sea surface temperature. The climate-stratigraphic method has been followed to separate Pleistocene sediments (Pushkar and Cherepanova, 2001, 2008; Polyakova, 1997), and identify complexes of diatoms corresponding to cooling and warming. The relationship between different groups of diatoms and the ice standing duration (Shiga and Koizumi, 2000) has been used to propose the distribution of and changes in ice cover in the Sea of Okhotsk over the past 21.5 kyr, and it has been found that the
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modern hydrological conditions of this region were established approximately 8 ka (Shimada et al., 2004, Okazaki et al., 2005). Recent studies on the paleoceanology and reconstruction of changes in the ice cover of the Sea of Okhotsk focused on changes in the paleoclimate, primary productivity, and the role of the ice cover in the development and variations of dominant microorganisms between glacial and interglacial periods (Okazaki et al., 2005; Sakamoto et al., 2006; Gorbarenko et al., 2010, 2012, 2014; Artemova et al., 2017, 2018; Sattarova and Artemova, 2015). However, the changes in diatom assemblages in response to changes in the southwestern Okhotsk Sea (SW OS) environment during the Late Pleistocene and Holocene periods have been poorly studied, despite the important role of diatoms as a sensitive proxy of variations of surface hydrology and sea ice coverage. Here, we report diatom assemblage data reflecting the S-W OS climate and environmental changes over the last 96 kyr, and correlate them with ice-rafted debris (IRD) data. The purpose of this work was to reconstruct the paleoceanographic conditions of the S-W OS during the Late Pleistocene and Holocene periods by analyzing the diatoms in a sediment core from the region. To achieve this goal, we studied the diatom assemblages that accumulated during the Late Pleistocene and Holocene periods (Marine Isotope Stages (MIS) 1-5). An age model for studied core was constructed, and the contents of CaCO3, total organic carbon (TOC), chlorin, and IRD were used to verify the diatom analysis results. Then the productivity and ice conditions of the S-W OS were reconstructed.
Standard 24 × 24-mm cover glasses and NORLAND synthetic gum with a refractive index of 1.56 were used to create permanent slides. The diatoms were identified and counted using a Micmed-6 microscope at a magnification of ×900. The number of diatom frustules was determined; and 300–350 frustules were identified for each slide to calculate the quantitative ratios of diatom taxa in the assemblages.
2. Oceanographic setting
CaCO3 = (TC-TOC) × 8.33
3.3. Chlorin content analysis The chlorin content of core samples was analyzed at the Pacific Oceanological Institute, Vladivostok, Russia (POI FEB RAS) using a Shimadzu UV-3600 spectrophotometer (each sample ca 1 g of dry sediment, every 1 cm, accuracy of up to 0.001 optical density units, wavelength of 665–666 nm) following the procedure proposed by Harris and Maxwell (1995). 3.4. Total organic carbon and CaCO3 analyses The total carbon (TC) and TOC contents of core samples collected every 2 cm were analyzed at the POI FEB RAS using an AN-752910N express analyzer. The CaCO3 and TC contents were determined following the acid and thermal decomposition (burning in a stream of oxygen) methods, respectively following the procedure reported by Gorbarenko et al. (1998). The CaCO3 content was calculated using a conversion factor for carbonate.
The Sea of Okhotsk is connected to the Pacific Ocean by numerous straits traversing the Kuril Island Ridge, the Sea of Japan by the La Pérouse Strait, and the Amursky Liman by the Nevelskoy and Tatar Straits. The Sea of Okhotsk is the coldest of the North West Pacific marginal seas. Overall, the current surface water circulation in the Sea of Okhotsk is cyclonic and, in general, consisted from the West Kamchatka Current, which carries warm Pacific water northward, and the East Sakhalin Current, which travels southward (Honjo, 1997; Fig. 1). In the Okhotsk Sea ice formation begins in November. The ice is thickest in March (0.8–1.0 m), and ice formation subsequently slows (Watanabe and Wakatsuchi, 1998; Martin et al., 1998). From April to June, the ice gradually melts and breaks apart. The hydrophysical and hydrochemical features of the surface water during spring create favorable conditions for phytoplankton blooms. The nutrient stock in surface layer forms in the Sea of Okhotsk during the autumn-winter convective mixing period, when deep water rises.
3.5. Oxygen and carbon isotopic ratios The isotopic compositions of oxygen and carbon (δ18O and δ13C) in the shells of Neogloboquadrina pachyderma sin., a species of planktonic foraminifera, were analyzed using a Finigan-MAT251 mass spectrometer without prior calcination following the standard technique in the laboratory of R. Tidemann (GEOMAR, Germany). 3.6. Magnetic susceptibility and moisture content The magnetic susceptibility and moisture content were measured every 2 cm along the length of the core during the cruise using an IMB-2 magnetic susceptibility kappameter with a special sensor for moisture (West Company Ltd., Kaliningrad). 3.7. Ice rafted debris
3. Materials and methods
IRD is the terrigenous component of sediment, i.e., it is the number of clastic grains in the 0.15–2.0 mm fraction per 1 g of dry sediment. Samples for determining the IRD content were collected every 3 cm. After wet sieving the sediment samples through a 0.15-mm sieve and the residue was dried and screened through a 2-mm sieve. The IRD content was measured in the remaining 0.15–2 mm sediment fraction. The IRD content analysis was described in detail by Vasilenko et al. (2019).
3.1. Materials Core GE 99-10-3 (48°18.7′N, 146°08′E) was obtained during the Russian-German cruise aboard the Pegasus Upland. The core was recovered from an area within the influence of the East Sakhalin Current at a depth of 1335 m, and had a length of 770 cm (Fig. 1). Lithological and micropaleontological analyses were conducted, and the magnetic susceptibility of the sediment, IRD, TOC, chlorin, and CaCO3, oxygen and carbon isotopic ratios, and the moisture content were determined following the methods described in this section.
3.8. Age model The age model of GE99-10-3 core was developed based on tephrochronology, the results of the δ18O analysis of foraminifera shells, the variations in magnetic susceptibility and moisture content, and the variations in the paleoproductivity proxies (chlorin, TOC, and CaCO3 contents; Fig. 2). Furthermore, three volcanic ash layers (K3, T, and Aso4;Derkachev et al., 2004, 2016; Derkachev and Portnyagin, 2013) were identified in the core, which were used as key age points (Figs. 2 & 3, Table 1).
3.2. Diatom analysis The diatoms in the sediments were insulated following a method developed at the Shirshov Institute of Oceanology RAS, which involves the use of a heavy potassium-cadmium liquid containing cadmium iodide (CdI2) and potassium iodide (KI) with the following ratio: H2O:CdI2:KI = 1:2.5:2.25 (Jousé, 1962; Diatoms of the USSR, 1974). 2
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Fig. 1. The location of core GE99-10-3. The blue arrows represent the surface currents, based on Chernyavskiy (1981). Numbers are standing for the follow straits: 1 Nevelskoy Strait, 2 - Tatar Strait, and 3 - La Pérouse Strait. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3.8.1. Position of boundaries between different marine isotopic stages The onset of MIS 1 (11.7 ka) was established according to the extreme lightening of the δ18O composition of planktonic foraminifera shells, the increasing contents of chlorin and TOC, and changes in the proxy parameters, which are characteristic for the onset of Holocene (Gorbarenko, 1996; Gorbarenko et al., 2002a,b, 2004; Figs. 2 & 3, Table 1). The onset of MIS 2 was stratigraphically located above the volcanic ash layer K3 and established following changes in the δ18O composition of planktonic foraminifera shells and the productivity proxies values (Fig. 2). The onset of MIS 3 corresponds to the transition from relatively low paleoproductivity proxies values during MIS 4 to relatively high values during MIS 3 (Fig. 2). The sharp peak in the chlorin content, which is the result of the increasing productivity during Dansgaard–Oeschger interstadial (DOI) 17, allows the position of this boundary to be elucidated (Fig. 2). The onset of MIS 4 was also determined by the general pattern of paleoproductivity proxies, particularly the transition from relatively high values during MIS 5 to relatively low values during MIS 4 (Fig. 2).
The division of MIS 1 into the period of early Holocene (11,7–6 ka) with dramatic changes in environmental conditions and sedimentation of the Sea of Okhotsk and the period of the late Holocene (6–0 ka) with relatively stable conditions (Gorbarenko, 1996; Gorbarenko et al., 2002a, 2002b; Barash et al., 2001). The boundary between these two time intervals is the sharp increase in the content of diatoms in the sediments of the Sea of Okhotsk with the formation of diatom ooze. The age of this boundary in the Sea of Okhotsk is defined as 6 ka and welldated (Gorbarenko, 1996; Gorbarenko et al., 2002ab, 2004, 2010). 3.8.2. Identification of the Dansgaard–Oeschger interstadials Gorbarenko et al. (2007, 2010, 2012) have demonstrated that the productivity of the Sea of Okhotsk increased simultaneously with the DOI and decreased during Dansgaard–Oeschger stadial (DOS). Thus, the productivity of the Sea of Okhotsk clearly responds to changes in the global climate on the millennial time scale, which was also reported in a study on ice cores collected in Greenland (Dansgaard et al., 1993). This allowed us to graphically correlate the productivity proxy records 3
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Fig. 2. Correlation of the changes in the productivity proxies (CaCO3, TOC, and chlorin contents), planktonic foraminifera δ18O record, and the moisture content and magnetic susceptibility of core GE99-10-3 with depth, along with the NGRIP δ18O record on the GICC05 age scale in the upper graph (North Greenland Ice Core Project Members, 2004; Wolff et al., 2010). The gray band denotes the position of the volcanic ash layers (K3, T, and Aso4). The ‘I’ terms with numbers in the upper section indicate the positions of the Dansgaard–Oeschger interstadials.
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Fig. 3. Age-Depth diagram for core GE 99-10-3. The ‘DOI’ terms with numbers in the upper section indicate the positions of the Dansgaard–Oeschger interstadials.
Table 1 Control points for the age model of core GE99-10-3. Control point
Dating method
Depth (cm)
Age (ka)
6 ka Onset of Onset of Onset of DOI 2 Onset of DOI 5 K3 DOI 6 DOI 7 T DOI 8 DOI 11 DOI 12 DOI 13 DOI 14 DOI 17 Onset of DOI 18 DOI 19́
Multiproxy analysis Multiproxy analysis Multiproxy analysis Multiproxy analysis Multiproxy analysis Multiproxy analysis Multiproxy analysis Volcanic ash layer Multiproxy analysis Multiproxy analysis Volcanic ash layer Multiproxy analysis Multiproxy analysis Multiproxy analysis Multiproxy analysis Multiproxy analysis Multiproxy analysis Multiproxy analysis Multiproxy analysis Multiproxy analysis Multiproxy analysis Multiproxy analysis Multiproxy analysis Multiproxy analysis Volcanic ash layer Multiproxy analysis
60 117 127 137 196 218 241 246–250 256 268 274–277 293 322 349 361 382 423.5 424 463 514 540 555 581 667 697–700 726
6 11.7 12.7 14.7 23.34 28 32.5 33 33.74 35.48 36.5 38.22 43.34 46.86 49.28 54.22 59.44 59.5 64.095 69.55 72.33 73.9 76.45 84.65 88 89.95
MIS 1 Younger Dryas (YD) Bølling-Allerød warming (B/A) MIS 2
MIS 3
DOI 19 Onset of MIS 4 DOI 20 DOI 21 Aso4 DOI 22
Note. DOI – Dansgaard–Oeschger interstadial.
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with that of the NGRIP δ18O content (North Greenland Ice Core, 2004), which provides a reliable tool for building high-resolution chronostratigraphy. The interstadials of long-lasting DOI cycles, which are before cycles 1, 8, 12, 14, 19, 20, 21, and 22 (Solé et al., 2007), are clearly established within the corresponding MIS and indicated by the peak values of the productivity proxies (chlorin, TOC) and moisture content, and the decrease in the magnetic susceptibility (Fig. 2). In addition to the above parameters, DOI 1 (onset of Bølling-Allerød (B/A) warming) was identified by the increase in the CaCO3 content (Fig. 2). The peak in the CaCO3 content is typical for Okhotsk Sea sediments that accumulated during B/A warming (Gorbarenko et al., 2002a, b). Additional age markers between the control points were obtained based on the graphical correlation of the chlorin content and magnetic susceptibility of the DO cycles in the δ18O NGRIP record (North Greenland Ice Core, 2004). Furthermore, increases in the TOC, CaCO3, and chlorin content and the minimum magnetic susceptibility values correlated with the peaks in the δ18O content corresponding to certain DOI (2, 5, 6, 7, 11, 13, 17, 18) (Fig. 2).
predominant: A. curvatulus (2.35–9%), R. hebetata f. hiemalis (0.9–15.9%), S. latimarginatus (17.3–2.3%), T. longissima (2.35–10%). The content of the tyhopelagic species Paralia sulcata also increased (3.8–28.9%). T. gravida (2.4–25%) was predominant in the near-ice group, and its proportion in this assemblage was the highest downcore (9–32%), followed by Bacterosira bathyomphala (3–11%). The proportion of sea-ice species (up to 10%), including Thalassiosira kryophila, Thalassiosira hyaline, Thalassiosira nordenskioeldii, Fragilariopsis cylindrus, and Fragilariopsis oceanica, also increased. Extinct redeposited diatoms were also founded in this assemblage. In the range of 24–18 kyr, the number of diatom frustules and proportion of the oceanic group in the sediment increased. During 18–12 kyr, the number of diatom frustules decreased. The content of S. latimarginatus increased, reaching 12.7–17.3% (Fig. 4). Assemblage 1 (since 11.7 ka) exhibited the highest abundance of diatom frustules (2.4–20.5 × 106frustules/1 g) with high species diversity. The oceanic species S. latimarginatus (5–18%), C. marginatus (13–35%), Neodenticula seminae (3.1–24.5%), and A. curvatulus (1–13%) were prevalent. The warm-water species Shionodiscus oestrupii (0.7–3.8%) and Thalassiosira pacifica (0.5–1.6%) were also observed. The share of oceanic species increased in accordance with the increasing diatom frustule content. The warm-water Japan Sea endemic species T. nitzhioides appeared since 5.6 ka. The share of sea-ice and near-ice diatoms decreased, and the content of P. sulcata was very small.
4. Results The following oceanic species were predominant in assemblage 5 (94–73.9 kyr): Shionodiscus latimarginatus (12–35%), Thalassiothrix longissima (10–18.5%), Coscinodiscus marginatus (12.5–33%), Rhizosolenia hebetata f. hiemalis (5.7–19.6%), and Actinocyclus curvatulus (3.4–20.5%). The diatom frustules content was 0.56–7.0 × 106/1 g (Figs. 4, 5). The content of near-ice species reached 7%, which was mainly due to the proportion of Thalassiosira gravida. Sea-ice flora are absent from this assemblage. The taxonomic composition of diatoms in assemblage 5 did not change significantly. However, the diatom frustule content fluctuated significantly. Three subassemblages can be identified based on changes in the concentrations of diatoms. In subassemblage 5c (94–93 kyr), the diatom content reached 2.5 × 106 frustules/1 g, and the proportion of oceanic species is small due to the high content of T. gravida (Fig. 4). In subassemblage 5b (93–85 kyr), the diatom content was lower than that of the previous assemblage (0.56–2.5 × 106 frustules/1 g). The proportion of oceanic species (35–65%) is lower than that of subassemblage 5c due to the presence of T. gravida (up to 7%) (Fig. 4, Plate 1). In subassemblage 5a (85–73.9 kyr), the increased diatom content of the sediment (2.8–7 x106 frustules/1 g) indicated that the conditions were more favorable for diatoms, and the proportion of oceanic species reached 92% (Fig. 4). The number of diatoms in assemblage 4 (73.9–59.5 kyr) was lower (0.19–1.12 x106 frustules/1 g). The diatom assemblages mainly contain cold-water oceanic species, such as R. hebetata (25–27%), T. longissima (6.6–25.6%), and S. latimarginatus (up to 21%). The percentage of nearice and sea-ice was 6% around 73.9–71.5 kyr, which continuously increased and reaching a maximum value of 21.8% at the 59.5 ka. However, its percentage sharply decreased about 69 ka. T. gravida is the predominant near-ice species. However, the content of cryophilic flora in assemblage 4 is insignificant (Fig. 4). In assemblage 3 (59.5–28 kyr), the number of diatoms increases (1.06–3.6 x106 frustules/1 g), along with the concentration of oceanic species increased. The number of near-ice diatoms decreased, constituting 2.2–10.4% of the total diatoms in the assemblage. The dominant species of this assemblage were A. curvatulus (16.4–3.4%), R. hebetata f. hiemalis (6.2–20.9%), C. marginatus (13–37%), S. latimarginatus (2.3–25.7%), and T. longissima (6.8–25.9%) (Fig. 4). Assemblage 2 (28–11.7 kyr,) is characterized by a sharp decrease in the number of diatom frustules per gram (up to 0.35 x106 frustules/1 g). The proportion of oceanic species is 35.7–70%, which is dominated by C. marginatus (13–37%). The following oceanic species were also
5. Discussion 5.1. End of MIS 5c-MIS 5b The low abundance and poor species composition of diatoms in the sediments during this period, in comparison with those of modern sediments, indicate that the climatic conditions were unfavorable at this time. The presence of arctic-boreal (including near-ice) species indicates a longer period of standing sea ice. The sea ice melted during summer, as indicated by the presence of P. sulcata in the assemblages (Figs. 4, 5). Predominance of typical Okhotsk cold-favoring species and high proportion of the near-ice group indicate that cold water masses prevailed in the S-W SO. Additionally, the climate was also cold. Based on the δ13C data, we can conclude that the terrigenous component was abundant in the supply of organic matter. The low contents of TOC and CaCO3 in the sediment also suggest low marine bio-productivity. However, according to the comparison of the IRD data collected for this period and the Holocene, the ice conditions were moderate and the winters were relatively mild. Within this period, the climate was cold, the sea regressed, and heavy terrigenous material was supplied to the sediments (Fig. 5). 5.2. MIS 5a During this period, the number of diatoms frustules, species diversity, and number of oceanic species increased. Moderately warm species were mainly present. The decrease in the percentage of sea-ice species indicates a reduction in the length of the winter seasons. The δ13C (Fig. 5) data suggest that marine productivity was high. During 84.5–81.5 kyr, the increasing proportion of R. hebetata f. hiemalis and TOC content indicated favorable climatic conditions with increased salinity and productivity in the surface waters of this area (Fig. 5). 5.3. MIS 4 The sediments of this period were characterized by low diatom abundance. The production of diatoms decreased due to the longer icecoverage season. The onset of glaciation conditions was gradual, which affected the ecological composition of diatoms. In the beginning of this 6
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Fig. 4. Distribution of the dominant species of diatoms, and the diatom content in core GE 99-10-3. The pink and dark blue bands denote periods with warm and cold environmental conditions, respectively. The light blue band denotes the period with slightly warmer environmental conditions during MIS 2. The red line marks the time at which the modern hydrological conditions began. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 5. Changes in the paleoproductivity proxies and environmental conditions of the southwestern region of the Sea of Okhotsk over time based on changes in the diatoms content in the sediment, and ratio of ecological groups, and the IRD, CaCO3, TOC, δ13C, and chlorin contents. The pink, dark blue, and light blue bands, and red line indicate the same periods as those in Fig. 4. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
period, oceanic flora was predominant. From 72 kyr, the diatom assemblages were dominated by sea-ice and near-ice species, particularly cryophilic species, which proliferated both in the ice and near the ice edge. This suggests the occurrence of relatively severe ice conditions with longer sea-ice standing seasons. This assumption is confirmed by the increase in the content of , decrease in the supply of marine
organics, and increase in the proportion of allochthonous bioorganic inputs, as indicated by the δ13C value. The presence of the brackish-water species P. sulcata, which reached a subdominant position in the diatom assemblages, suggests the occurrence of active warming and ice-melting periods. Generally, the data indicated a cooling climate. The peak content of extinct species 8
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Plate 1. Diatom assemblages in the sediments from the southwestern part of the Sea of Okhotsk. 1–Actinocyclus ochotensis Jousé. 2–Actinocyclus curvatulus Janisch. 3– Thalassiosira gravida Cleve, Shionodiscus latimarginatus (Makarova) Alverson. 4–Rhizosolenia hebetata Bailey. 5–Bacterosira bathyomphala (Cleve) Syvertsen & Hasle. 6– Coscinodiscus oculus-iridis (Ehrenberg) Ehrenberg. 7–Shionodiscus latimarginatus (Makarova) Alverson. 8–Thalassiosira gravida Cleve. 9– Coscinodiscus marginatus Ehrenberg. 1011– Neodenticula seminae (Simonsen et Kanaya) Akiba et Yanagisawa. 12– Thalassiothrix longissima Cleve et Grunow. 13–Thalassionema nitzschioides (Grunow) Mereschkowskii.
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indicated that the sea level regressed. The absence of Japanese Sea species point to that there was no interaction between the Seas of Okhotsk and Japan.
environment and the enhancement of primary bioproductivity. However, as these conditions are unfavorable for the diatoms, there were few diatom products. The high bioproductivity was attributed to the development of carbonate microfossils.
5.4. MIS 3
5.7. Younger Dryas
The increase in the number of diatom frustules and content of oceanic species in the sediments indicates that, in this period, the Sea of Okhotsk experienced different climatic conditions. The ecological composition of diatoms at this time reflects a relatively warm climate, however, interglacial environmental conditions were not reached. The TOC content indicates that the supply of organic material to the bottom was slightly higher than that in the previous cold period. The lightening δ13C isotopes also indicate an increased supply of autochtonous organics. The number of near-ice and sea-ice diatoms decreased, reflecting a decrease in the ice coverage of the S-W OS. The salinity of the surface layer may have been higher than it is today. During this period, two regional warming events occurred at 58–49 and 45–36 kyr. The warming, accompanied by active ice melting, increased the diatom vegetation period. The lightening of the δ13C isotope composition indicates an increase in the supply of marine organics. Furthermore, the absence of CaCO3 confirms the predominance of diatoms in the phytoplankton (Fig. 5). From 36 to 28 ka, the number of diatoms decreased, the proportion of oceanic species sharply reduced, and the proportion of near-ice flora increased. Severe climatic conditions occurred, as indicated by the decrease in all productivity proxies, as well as the significant increase in the IRD content during this period (Fig. 5). Our data suggest that the severe climatic conditions, which are usually noted as characteristic of MIS 2, began earlier in this region, commencing 34 ka.
During this period, the cooling event occurred. This was reflected in the increase in the contents of ice and glacial species in the diatom complex. The number of diatoms was low, which was because the previous deglacial warming was interrupted by strong, short-term cooling, and an increase in the duration of the winter season. The TOC content decreased alongside the decrease in the concentrations of chlorin and CaCO3, confirming the return to glacial and severe environmental conditions, and a decrease in primary productivity (Fig. 5). 5.8. Early Holocene (11.7–8.3 kyr) This period marked the beginning of Holocene warming (Termination 1B). The concentration of near-ice species in the glacial assemblages decreased, and the proportion of sea-ice diatoms was lowest. This was due to the significant warming and decrease in the extent of sea ice. The low abundance of diatoms was likely due to the low input of nutrients, and their growth was limited by the low amount of silicon. An interesting local event occurred at 11.7–11 kyr, which was associated with the strong desalination of the sea surface layer due to the increased ice melting. This was marked by the synchronous peaks in the growth of P. sulcata and the IRD content. The sharp increase in the proportion of R. hebetata following this event indicated an increase in salinity in the study area until 6 ka.
5.5. MIS 2
5.9. The last 8.3 kyr
The severely cold climate conditions and low bio-productivity continued until 24.5 ka. From 24.5 to 18 ka, the composition of diatoms reflects some environmental warming in the S-W OS. The number of diatom frustules and the proportion of oceanic species both increased. According to our diatom data, from 18 to 15 ka, the hydrological conditions were significantly different to those observed today. The diatom assemblages were unique and have not been found in modern deep-water sediments from the Sea of Okhotsk. Diatom assemblages with glacial species, including B. bathyomphala, Thalassiosira antarctica, and T. gravida, and cryophilic species are typical for sediments in the Arctic seas (Polyakova, 1996; Tsoy et al., 2009). The diatom complex indicates prolonged sea ice standing, a reduction in the length of summer seasons, and temperature conditions close to those currently observed in the Arctic regions. The long sea ice-standing seasons, decrease in the input of the both Pacific waters and Amur River inflow, which provides a large amount of necessary for the development of diatoms nutrients to the Sea of Okhotsk (Seki et al., 2004a, 2004b), reduced the productivity of diatoms. The sharp decline in CaCO3 and TOC confirms the occurrence of severe glaciation conditions, known as the cooling event Heinrich 1 (Gorbarenko et al., 2014). The increase in the volumes of sea ice was confirmed by the spike in the IRD record. The decrease in the sea level was reflected in the appearance of redeposited diatom species from the eroded, older Cenozoic sediments.
This period is characterized by the rapid development of diatom flora and significant increase in their abundance. The increased diatom production, flow of organic matter, and nutrient inputs from land were confirmed by the chlorin and δ13C data. During this period, the hydrological conditions of the Sea of Okhotsk were turned to those experienced today. The resumption of the interactions between the Seas of Japan and Okhotsk, and the influx of warm and saline waters from the Soya Current were indicated by the presence of Thalassionema nitzschioides in the sediment, which migrated from the Sea of Japan approximately 5.6 ka, later then mentioned in Shimada et al. (2004) for off Hokkaido Island. The appearance of N. seminae over the last 6 kyr indicates the enhanced input of Pacific surface water with the onset of modern Sea of Okhotsk hydrology. 6. Conclusions The diatom species composition and content of diatom frustules in the sediments of the S-W OS, during the Holocene and Late Pleistocene periods reflect the global and regional climatic and environmental changes over the last 94 kyr. During studied period prolonged sea icestanding season and extending sea-ice cover led to lowered diatoms productivity and vice versa. Sea level regression led to the absence of interactions between the Seas of Okhotsk and Japan about 75 ka. During 58–49 and 45–36 kyr, regional active warming events occurred with higher productivity and an increase in the supply of marine organics. During 36–28 kyr, severe climatic conditions were experienced, and the productivity of marine phytoplankton and influx of organic matter from the shore were low. Local cooling began in 34 kyr. From 18 to 15 kyr, a very long season with an ice-covered sea occurred, and the environmental conditions of this region were similar to those currently observed in the Arctic. The sea level was also significantly lower, which is likely due to climate aridization and terrestrial glaciation. During B/A, a period of warming and active ice melting with a
5.6. Bølling-Allerød During this period, the warming, the longer summer seasons, and the melting of great volumes of sea ice were reflected in the growth of oceanic species and the increased share of the brackish-water, lowsalinity-tolerant species P. sulcata (reaching 28.9%, the highest in studied sediments). During this period, there were sharp and significant peaks in the contents of productivity indicators (TOC, chlorin, and CaCO3). This confirms the favorable climatic conditions of the 10
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great flood and desalination of seawater was experienced alongside an increase in productivity. During YD, glacial conditions returned, while significant warming occurred from 11.7 to 8.3 kyr. During 11.7–11 kyr, large volumes of floating ice began to drift into the study area and the seawater was strongly desalinated, which prevents the development of diatom flora. In the last 8.3 kyr, the productivity, organic matter content and nutrient inputs increased. Modern hydrological conditions were established 5.6 kyr, water exchange between the neighboring basins of the Seas of Japan and Okhotsk commenced, and the influx of Pacific water also increased.
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Declaration of Competing Interest We declare that we have no conflict of interest. Acknowledgements The authors are extremely grateful to Dr. M. Malyutina and Prof. Dr. A. Brandt for the invitation to join the expedition KuramBio II supported by BMBF Grant no. 03G0250A to Angelika Brandt. This work was supported by the Russian Foundation for Basic Research (grant no. 19-05-00663 A), the project of the Ministry of Science and Education of Russia (grant no. АААА- А17-117030110033-0). This is KuramBio publication # 59. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.pocean.2019.102215. References Artemova, A.A., Gorbarenko, S.A., Vasilenko, Yu.P., Shi, X., Liu, Y., Chen, M.-T., 2017. Paleoceonography changes in the Okhotsk Sea during Late Pleistocene and Holocene according to diatoms. Quat. Int. 459, 175–186. Artemova, A.V., Sattarova, V.V., Vasilenko, Yu.P., 2018. Diatom distribution and geochemical evidence in the Holocene Kurile Basin sediments (Sea of Okhotsk). Deep Sea Research Part II: Topical Studies in Oceanography. https://doi.org/10.1016/j.dsr2.2017.12.019. Barash, M.S., Bubenshchikova, N.V., Kazarina, G.Kh., Khusid, T.A., 2001. Paleoceanograpfic studies in the central part of the Okhotsk Sea during the last 200 kyr (on the basis of micropaleontological data). Okeanologiya 41 (5), 755–767 in Russian. Chernyavskiy, V.I., 1981. Circulation systems of the Okhotsk sea. Izvestiya TINRO 105, 13–19 in Russian. Dansgaard, W., Johnsen, S.J., Clausen, H.B., Dahl-Jensen, D., Gundestrup, N.S., Hammer, C.U., Hvidberg, C.S., Steffensen, J.P., Sveinbjörnsdottir, A.E., Jouzel, J., Bond, G., 1993. Evidence of general instability of past climate from a 250-kyr ice-core record. Nature 364, 218–220. Derkachev, A.N., Nikolaeva, N.A., Gorbarenko, S.A., Portnyagin, M.V., Ponomareva, V.V., Nürnberg, D., Sakamoto, T., Iijima, K., Liu, Y., Shi, X., Lv, H., Wang, K., 2016. Tephra layers of in the quaternary deposits of the Sea of Okhotsk: Distribution, composition, age and volcanic sources. Quatern. Int. 425 (15), 248–272. Derkachev, A.N., Portnyagin, M.V., 2013. Marker tephra layers in the late quaternary deposits of the Sea of Okhotsk as evidence of catastrophic eruptions in the Nemo caldera complex (Onekotan Island, Kuril Islands). Stratigr. Geol. Correl. 21 (5), 553–571. Derkachev, A.N., Nikolaeva, N.A., Gorbarenko, S.A., 2004. The peculiarities of supply and distribution of clastogenic material in the Sea of Okhotsk during Late Quaternary. Russ. J. Pacif. Geol. 23 (1), 37–52. Diatoms of the USSR (fossil and modern), Nauka, Leningrad, 1974 (In Russian). Gorbarenko, S.A., 1996. Stable isotope and lithological evidence of late-glacial and Holocene oceanography of the Northwestern Pacific and its marginal seas. Quat. Res. 46, 230–250. Gorbarenko, S.A., Artemova, A.V., Goldberg, E.L., Vasilenko, Y.P., 2014. The response of the Okhotsk Sea environment to the orbital-millennium global climate changes during the Last Glacial Maximum, deglaciation and Holocene. Global Planet. Change 116, 76–90. Gorbarenko, S.A., Chekhovskaya, M.P., Souton, J.R., 1998. Detailed environmental changes of the Okhotsk Sea central part during Last Glaciation Holocene. Okeanologiya 38 (2), 305–308 In Russian. Gorbarenko, S.A., Goldberg, E.L., Kashgarian, M., Velivetskaya, T.A., Zakharkov, S.P., Pechnikov, V.S., Bosin, A.A., Psheneva, O.Yu., Ivanova, E.D., 2007. Millennium scale environment changes of the Okhotsk Sea during last 80 kyr and their phase relationship with global climate changes. J. Oceanogr. 63 (4), 609–623. Gorbarenko, S.A., Harada, N., Malakhov, M.I., Velivetskaya, T.A., Vasilenko, Y.P., Bosin, A.A., Derkachev, A.N., Goldberg, E.L., Ignatiev, A.V., 2012. Responses of the Okhotsk Sea environment and sedimentology to global climate changes at the orbital and millennial scale during the last 350kyr. Deep-Sea Res. II 61–64, 73–84.
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